Re: Deadlock on the mm->mmap_sem

[Manfred Spraul]

Andrea Arcangeli wrote:
> 
> > I'll write a patch that moves the locking into the coredump handlers,
> > then we can compare that with Andrea's proposal.
> 
> Ok.
>
I've changed my mind:

Modifying the mmap_sem is a better solution for 2.4 than integrating the
locking into elf_core_dump.

My patch copies the vm areas into a list (under down_write()) and calls
up_write(), but I found 2 races:
* the kernel must not touch VM_IO memory. Another thread could call
"munmap(), mmap(,VM_IO)".
* If another thread calls munmap(), my coredump handler would abort
dumping due to the resulting pagefault.

The proper solution would be using a page table walker in elf_core_dump
(similar to access_process_vm()), everything under down_write().

But that would be a large rewrite. I'm aware of at least 4 users who
want such a page table walker: map_user_kiobuf, access_process_vm,
singlecopy pipe (not merged), elf_core_dump.

--
	Manfred